The present invention relates generally to the field of ground fault interrupt protection for electrical power supplies and in particular to a new and useful ground fault protection for the secondary coil of a high voltage transformer. The ground fault protection is especially useful for power supplies used to power lighting applications through a high voltage transformer, such as a neon lighting display.
Ground fault protection circuits for lighting display power supplies are generally known in the art. U.S. Pat. No. 5,751,523, for example, discloses a power supply for neon lamps having a transformer with a return path that is separate from the earth ground, which permits detection of a fault current. The primary coil is connected to a power source. A load, such as gas discharge tubes, is connected across the secondary coil end terminals. The mid-point of the secondary coil of the power supply is connected to one side of a secondary ground fault protection circuit and to ground. The secondary ground fault protection circuit is also connected to the primary coil terminals. The secondary ground fault protection circuit includes a relay for breaking the connection to the AC power source connected to the primary coil when a ground fault is detected.
Other patents disclosing power supplies having fault protection include U.S. Pat. Nos. 5,387,845, 5,841,239, 5,241,443, 4,507,698 and 3,666,993.
In certain cases, different gas pressures and types of gas discharge tubes, such as neon gas tubes, can present different amplitude loads to power supplies. In the case of a system where the color generated by a gas discharge tube may be changed by changing the voltage amplitude, frequency and/or duty cycle supplied to the tube, a power supply which is operating safely when driving tubes generating a yellow color may be subject to faults when a blue color is generated using the same power supply instead.
There is a need for a ground fault protection circuit for a power supply having a sensing circuit which can detect ground faults, a bad sensing circuit and floating grounds while distinguishing between different amplitude loads.
It is an object of the present invention to provide a secondary ground fault protection for a power supply transformer having fault sensing circuitry capable of indicating and reacting to different fault conditions.
It is a further object of the invention to provide a secondary ground fault protection which can verify the function of the sensing transformer and the security of the earth ground connection.
It is yet another object of the invention to provide a secondary ground fault protection for terminating both hardware high voltage generation and power supply software power generation instructions.
A further object of the invention is to provide a ground fault protection for a power supply used to power a changing load.
Accordingly, a secondary ground fault protection for a high voltage power supply is provided having a high voltage transformer with a center tapped secondary coil. The primary coil of a monitoring transformer is connected to the secondary coil at the center tap, which is approximately the midpoint of the secondary coil. The power supply load is connected across the end terminals of the secondary coil.
The monitoring transformer is connected between the center tap and an earth ground on the primary coil side and between sensing circuitry and a digital ground on the secondary side. The sensing circuitry includes sub-circuits that can generate outputs indicating the presence of faults, including a floating ground, excessive fault current or a defective sensor circuit. The sub-circuit outputs can be connected to cause specific actions in response to a particular fault, such as terminating the high voltage generation in response to an excessive fault current. The ground fault detection circuit includes inputs for indicating what load is being powered by the power supply. The ground fault sensing is made more accurate and effective by using a threshold comparison voltage corresponding to the load being powered.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.